I. Field of the Invention
The present invention relates generally to mobile telephone systems, and more particularly to a method and system for transmit power control of an outdoor antenna unit in a mobile telephone system by a coupled wireless device. The present invention is most applicable to wireless communications systems using code division multiple access (CDMA) modulation techniques where power control is very important.
II. Related Art
Mobile telephone systems allow customers to establish communication links or place telephone calls from wireless devices such as portable or hand-held mobile phones. Calls initiated or received by wireless devices used in such systems are processed by a wireless network. One type of wireless network is a terrestrial cellular communication system communicating via a series of base stations and ground-based antennas that operate in the 800-1900 MHz range. Cellular communication systems limit the user to communication within a cell, which comprises a geographical service area to which the base station antennas can transmit. Users can move from cell to cell through known hand-off procedures that transfer calls from one cell to another. However, if no base station is within range of the mobile transmitter, such as in a rural area, a user cannot use the mobile telephone service.
Developments in mobile telephone system technology have led to wireless communication systems or networks that can transfer signals using a Low Earth Orbit (LEO) satellite system. The satellite systems can transmit and receive signals in rural areas as well as cities through the beams they project, and a user does not need to be within close range of a ground-based antenna. As a result, satellite communication systems are not limited to major cities as are cellular networks. In addition, each LEO satellite is capable of carrying a large number of user transmissions simultaneously. Various satellite access schemes such as time division multiple access (TDMA) and code division multiple access (CDMA) allow concurrent access to LEO satellites by a large number of users.
The number of users that can be serviced by a wireless communication system, the system capacity, increases if the power output from each user""s wireless device is decreased to the minimum power needed for quality transmission, and overhead or nontraffic messages or channel activity is reduced. This is the result of decreasing mutual interference between users, which is especially important in limited power environments such as CDMA type communication systems. However, if the power of a user""s signal becomes too low, the quality of service for that user becomes unacceptable. So, there is a desire to maintain as high a power level as possible to have higher quality service.
Thus, the number of users that may be provided service is increased by maintaining overhead power levels and each individual user""s signals at the minimum levels needed for optimum performance. Therefore, the power output of wireless device transmissions are generally controlled using one or more power control methods to minimize interference and maximize communication link quality. Techniques for power control are discussed for example in U.S. Pat. No. 5,383,219, entitled xe2x80x9cFast Forward Link Power Control In A Code Division Multiple Access System,xe2x80x9d issued Jan. 17, 1995; U.S. Pat. No. 5,396,516, entitled xe2x80x9cMethod And System For The Dynamic Modification Of Control Parameters In A Transmitter Power Control System,xe2x80x9d issued Mar. 7, 1995; and U.S. Pat. No. 5,267,262, entitled xe2x80x9cTransmitter Power Control System,xe2x80x9d issued Nov. 30, 1993, which are incorporated herein by reference. In addition, see U.S. patent application Ser. No. 09/164,384 filed Sep. 30, 1998 entitled xe2x80x9cSystem And Method For Optimized Power Controlxe2x80x9d; and Ser. No. 08/908,528, filed Aug. 7, 1997, entitled xe2x80x9cMethod And Apparatus For Adaptive Closed Loop Power Using Open Loop Measurements,xe2x80x9d which are incorporated herein by reference. The result of using such power control techniques is that the communication system is capable of carrying substantially the maximum number of individual user transmissions simultaneously.
Wireless devices, also referred to as user terminals, in current wireless communications systems may be any of several different types. One type is the portable unit, which is a hand-held device carried by the user and requires no external power source or antenna system. Another type is the mobile unit or station, which is typically fixed in a vehicle and operates like a desk type phone. A mobile unit has a separate unit (or xe2x80x9cboxxe2x80x9d) that is mounted in the vehicle and contains most of the transmitting and receiving circuits or hardware. A hand-held unit such as a phone handset, containing a keypad, speaker and microphone, is connected by one or more cables, conductors, or connectors to the box. A cradle is provided for supporting the handset unit when it is not in operation or is being used in a xe2x80x9chands freexe2x80x9d mode. The box in turn is connected by a cable to an externally mounted xe2x80x9coutdoorxe2x80x9d antenna unit, which transmits and receives signals via a satellite or terrestrial cellular communications system or a base station or gateway.
A third type of user terminal combines the features of both a portable unit and a mobile unit. This type uses a hand-held device that can be used as a standalone unit away from the vehicle, and can be connected to a vehicle mounted assembly sometimes called a xe2x80x9ccar kit,xe2x80x9d for use in the vehicle. The car kit uses an external or outdoor unit (ODU) with an outdoor antenna to accommodate communications for the wireless device. A primary advantage of this combination unit or arrangement is that when the wireless device is used in the vehicle it can utilize additional power provided by the vehicle mounted electronics to establish a better and stronger communications link with satellite transceivers. It also allows conservation of internal battery power, drawing on vehicle provided power instead.
Satellite telephone systems are particularly sensitive to outdoor or external antenna unit matching at the mobile unit due to potential path losses and a resulting difficulty in power control. For optimum performance, the power output of the outdoor unit must be configured with specific phone transmission characteristics or circuits in mind. At present, a given car-kit is designed in the factory to function with certain characteristics for phones with which it is to be connected. Once this designing takes place, the optimum performance for a phone is limited to being used with certain car kits or outdoor antenna units, such as specific models or manufacturers, having closely matched characteristics.
This clearly sets certain constraints for mobile units or hand-held phones used with car kits in order to provide a closer match between the power output desired by the phone power control systems or methods, and the power actually being delivered by the outdoor unit. Yet, in the actual marketplace it is not uncommon that a user may have more than one portable phone or have one that can be used both as a standalone unit and as a mobile phone when placed in a cradle mounted in a vehicle. Also, a user may commonly upgrade his or her phone as new models come on the market. Therefore, the mobile unit employed with a car kit may change permanently or on a transient basis, creating potential undesirable power matching problems.
One goal of the present invention is to allow a variety of hand-held wireless devices to be used with a given vehicle mounted outdoor antenna unit or car-kit, while maintaining a desired level of accuracy for the power output by the outdoor unit. Here, a given outdoor antenna unit is not calibrated against specific hand-held device characteristics, but allowed to adjust in response to changing characteristics or power requirements for a device to which it is coupled. For improved or optimum performance, however, the outdoor antenna unit and the hand-held wireless device connected to it need to communicate transmit power levels and requirements.
What is needed is a system and method for the hand-held device to inform the outdoor unit of an appropriate transmit power level to produce or use in transferring signals, using a control signal. Furthermore, what is needed is a system and method for allowing the outdoor unit to adjust its transmit power level based on the control input, to better match or coordinate with the desired performance of the wireless device.
The present invention is a novel system and method for controlling transmit power in a car kit outdoor unit by a coupled wireless device. With the present invention, transmit power is controlled by obtaining an output power measurement of the information signal transmitting from an external antenna unit, also referred to as an outdoor unit. The output power measurement is provided to a hand held wireless device via a feedback loop to adjust the initial and subsequent power of information signals transmitting from the hand held device.
The system of the present invention includes an outdoor unit, a hand held device, and any other units needed for transmission. An exemplary additional unit for transmission may be a cradle for supporting the hand held device, and for interfacing with the outdoor unit. The antenna unit of the present invention includes components for transmitting information signals and components for obtaining an output power measurement of information signals immediately prior to transmission. The hand held device comprises one or more transmit and receive components and logic that can interpret the output power measurement and adjust transmission power of the information signal, or outdoor unit, accordingly.
The method of the present invention includes obtaining an output power measurement, modulating the power measurement onto a carrier, and transmitting the modulated measurement signal to the hand held device. The method of the present invention also includes receiving the modulated measurement signal by a cradle or directly by hand held device, demodulating the measurement signal, and adjusting the transmit power of the information signal.
The system and method of the present invention allow control of the initial transmit power of the information signal from the wireless device in order to limit the output power of the signal transmitted from the device to the power needed for high quality transmission, in accordance with known constraints on emission levels, and in view of desired or acceptable xe2x80x9cother devicexe2x80x9d interference levels. Limiting the output power from an outdoor unit, or the hand held wireless device, allows a maximum number of individual user transmissions to be carried by in a wireless communication system, and avoids potential problems with emission standards.